The structure of a conventional wire-wound resistor is shown in FIG. 1 and FIG. 2. In FIG. 2, the wire-wound resistor 10 comprises a ceramic rod 11, wherein the right end and left end of the ceramic rod 11 are respectively connected to a first iron cap 121 at the right end and a second iron cap 122 at the left end, and a wound metal wire 13, which is helically wound around the ceramic rod 11 along the circumference of the ceramic rod 11 from a wire head 131 on the first iron cap 121 to a wire tail 132 on the second iron cap 122, the wire head 131 of the wound metal wire 13 is subsequently soldered and fixed onto a wire-head soldering point 1311 of the first iron cap 121 by an electric soldering machine, and the wire tail 132 of the wound metal wire 13 is soldered and fixed on a wire-tail soldering point 1321 of the second iron cap 122, and then a first lead wire 141 and a second lead wire 142 extend respectively from the right of the first iron cap 121 and the left of the second iron cap 122 to form a conventional wire-wound resistor.
A wire-wound resistor is not the mainstream as to conventional surge-resistant resistors. When the transient energy of a surge wave is more than 100 watts, the surge loosens a certain proportion of the wound wires at the soldering points, which affects the surge resistance. In other words, for a conventional wire-wound resistor, when a wire head or a wire tail is soldered onto an iron cap obliquely or the soldering penetrates too deeply or not deep enough (as shown in FIG. 1, the wire tail 932 of the wound wire 93 is soldered obliquely at the soldering point 9321 of the second iron cap 922 on the ceramic rod 91), the contact resistance between the soldering point and the iron cap increases because the soldering points are soldered poorly (for example a soldering machine). Therefore, a surge event may loosen the soldering points and a certain failure rate of the soldering points of a wire-wound resistor may ensue. The failure rate of the surge-resistant soldering points of a conventional wire-wound resistor is approximately 10 ppm. Because the failure rate of the aforementioned surge-resistant soldering points is still high, the wire-wound resistor industry is eagerly looking for a surge-resistant wire-wound resistor which has highly reliable surge-resistant soldering points.
Owing to the aforementioned drawbacks of prior arts, the present invention provides a highly reliable wire-wound resistor to decrease the failure rate of the surge-resistant soldering points and to improve the surge-resistance reliability.